Diving without any scuba equipment is restricted to short time periods during which people can hold their breath.
Diving using scuba equipment makes it possible to swim underwater for a long time. However, diving with scuba equipment requires much training and experience. Therefore, it is hard for people in general to dive or swim with scuba equipment. Therefore, in order to swim underwater or to obtain living space underwater, for example, a station or a dome is constructed underwater, and the interior of the dome is used as a living space. Further, Japanese Utility Model Unexamined Publication No. 49-17000 discloses an apparatus which can make it possible to swim and to behave in a desired manner in an underwater dome. The apparatus includes means for supporting the dome at a desired depth in the sea, a gate through which users can come into and go out from the dome, and means for supplying air of a pressure higher than the water pressure at such depth. However, such apparatus has a complicated construction and no known apparatus has been proposed yet which can easily and readily provide people with living space underwater.